The present invention relates to novel pyrone derivatives of the general formula 
in which X1 is halomethyl, with the exception of monobromomethyl and dibromomethyl, and a process for their preparation.
Pyrone derivatives of the general formula I such as, for example, 2,6-bis(chloromethyl)-4-pyrone can be important intermediates for the preparation of crown ethers (W. Lxc3x6we et al., J. Heterocyclic Chem., 34, 1173 (1997)).
Until now, only 2,6-bis(bromomethyl)-4-pyrone and 2,6-bis(dibromomethyl)-4-pyrone and a process for their preparation were known (Lxc3x6we et al., ibid). In this process, 2,6-dimethyl-4-pyrone is brominated using N-bromosuccinimide/dibenzoyl peroxide in tetrachloromethane. A disadvantage of this process is that a mixture of various products is obtained, from which the said compounds can only be isolated in very low yield.
The object of the present invention was therefore to make available compounds which are suitable for the synthesis of crown ethers and a process for their preparation.
According to the invention, this object is achieved by the preparation method according to claim 1 and the novel pyrone derivatives according to claim 6.
The process according to the invention for the preparation of the pyrone derivatives of the general formula 
in which X1 is halomethyl is carried out by treating an acetoacetic acid derivative of the general formula 
in which X1 has the meaning mentioned and X2 is chlorine or bromine, first with a Lewis acid and then converting it into the desired product of the general formula I using water.
Halomethyl is understood here and below as meaning trihalomethyl, dihalomethyl or monohalomethyl. In this case, trihalomethyl and dihalomethyl may contain both identical and different halogen atoms. Examples of trihalomethyl are trifluoromethyl and tribromomethyl, examples of dihalomethyl are dichloromethyl, dibromomethyl, difluoromethyl and examples of monohalomethyl are fluoromethyl, chloromethyl, bromomethyl or iodomethyl. Suitable halomethyl is, for example, also difluoromonochloromethyl, dibromomonochloromethyl and dichloromonofluoromethyl.
Suitable Lewis acids are scandium salts such as, for example, scandium trifluoromethanesulphonate, scandium sulphate, lanthanide salts such as, for example, lanthanum trifluoromethanesulphonate, yttrium salts or BF3 xc2x7O(C2H5)2. The Lewis acid preferably employed is scandium trifluoromethanesulphonate.
The Lewis acid is advatageously employed in an amount from 0.01 to 3 mol, preferably in an amount from 0.05 to 0.5 mol, per mole of acetoacetic acid derivative of the general formula II.
The reaction is preferably carried out in a halogenated organic solvent. Halogenated organic solvents which can be employed are halohydrocarbons such as dichloromethane, chloroform, tetrachloromethane, 1,1,1-trichloroethane or 1,2-dichloroethane. Dichloromethane is particularly preferred.
The reaction is expediently carried out at a temperature of xe2x88x9220 to 60xc2x0 C., preferably at a temperature of 0 to 25xc2x0 C.
The reaction in the first and second stage is expediently carried out under an inert gas atmosphere such as, for example, under a nitrogen atmosphere.
After a customary reaction time of 1 to 8 days, the pyrone derivatives of the general formula I can be isolated by means of customary working-up methods.
The pyrone derivatives of the general formula I 
in which X1 is halomethyl, with the exception of 2,6-bis(bromomethyl)-4-pyrone and 2,6-bis(dibromomethyl)-4-pyrone (X1=bromomethyl or dibromomethyl), are novel compounds and likewise form subject-matter of the invention. Preferred pyrone derivatives of the general formula I are 2,6-bis(chloromethyl)-4-pyrone, 2,6-bis(fluoromethyl)-4-pyrone, 2,6-bis(iodomethyl)-4-pyrone, 2,6-bis(trifluoromethyl)-4-pyrone, 2,6-bis(tri-chloromethyl)-4-pyrone, 2,6-bis(tribromomethyl)4-pyrone, 2,6-bis(triiodomethyl)-4-pyrone and 2,6-bis(difluoromonochloromethyl)-4-pyrone.
A further subject-matter of the invention is the preparation of acetoacetyl derivatives of the general formula 
in which X1 has the meaning mentioned and R is a C1-6-alkyl group. Some compounds of this type are disclosed in CH-A-602 542. However, the synthesis described there is carried out in anhydrous hydrogen fluoride and is problematical owing to the corrosiveness and toxicity of the said compound.
According to the invention, an acetoacetic acid derivative of the general formula 
in which X1 and X2 have the meaning mentioned, is converted in the presence of a Lewis acid using a benzene derivative of the general formula 
in which R is a C1-6-alkyl group, into the final product according to formula III.
C1-6-Alkyl is understood here and below as meaning both linear and branched alkyl groups having up to 6 carbon atoms such as, for example, methyl, ethyl, propyl, isopropyl, butyl, isobuyl, tert-butyl, pentyl and its isomers and hexyl and its isomers.
This reaction is advantageously carried out at a temperature of 20 to 30xc2x0 C.
Scandium trifluoromethanesulphonate is preferably used as Lewis acid.
After a customary reaction time of about 2 days, the acetoacetyl derivatives of the general formula III can be isolated by customary working-up methods.
The acetoacetyl derivatives of the general formula 
in which X1 has the meaning mentioned and R is a C1-6-alkyl, with the exception of xcex3-bromoacetoacetylmesitylene and xcex3,xcex3,xcex3-trichloro-acetoacetylmesitylene, are novel compounds and likewise form subject-matter of the present invention. The examples below illustrate the implementation of the process according to the invention and the preparation of the inventive compounds.